Origin of the North Atlantic Clod Blob Revisited

The cold blob refers to an observationally unprecedented, gyre-scale, record-breaking cold of mean surface temperature over the subpolar North Atlantic. Its anomalous cold feature goes against the rising trend of global mean surface temperature in the context of a warming climate. Observations show that the Atlantic cold blob emerged in early 2014 and can penetrate deeper into the ocean interior beyond 500m depths. A sudden drop in upper ocean heat content is associated with an accumulative increase in freshwater content. Prior works pointed out that intense surface forcing during two consecutive winters was a primary driver. We hypothesize that surface forcing alone is insufficient for the cold blob to persist. Our analysis shows, for the first time, that variations in the net surface heat fluxes cannot explain the decline in upper ocean heat content during 2014–2017. Therefore, surface forcing fails to explain the origin of the cold blob. To investigate alternative mechanisms, non-assimilative simulations based on a coupled ocean-sea ice model (GFDL MOM5/SIS1) with two different atmospheric forcings (MERRA2 and ERA-interim) are employed to examine the transports of mass, heat, and freshwater within the cold blob area. Initial diagnosis verified that both model runs can reproduce the cold blob characteristics at similar magnitudes to Argo observations. Model results show a decreasing trend of heat transport at the southern boundary, implying that reduced poleward ocean heat transport likely accounts for the formation and persistence of the cold blob. This cooling signal from the south is accompanied by a freshening signal. Changes in the residual heat fluxes suggest that reduced warming for the subsurface layer at 100–700 m depths apparently occurred since 2006 before turning into enhanced cooling during late 2013. Variations in the residual freshwater fluxes remain positive for the entire past decade and subsequently result in an accumulative surplus of freshwater content in this area. The model run with incorporated Greenland meltwater estimates sheds light on the relative contribution of meltwater advection. To a great extent, Greenland meltwater can amplify the freshening tendency in the subpolar North Atlantic by approximately up to 200% during the present decade. In the long run, upper ocean cooling and freshening would lead to increased stratification and reduced mixing with deeper waters, therefore enhancing the likelihood that the subsurface cold blob persists.